1. Field of the Invention
The present invention relates, in general, to a method for forming a photoresist film pattern on a wafer substrate by a lithography process and, more particularly, to the use of a silylation photoresist film in forming a fine pattern.
2. Description of the Prior Art
Necessary to form a pattern in a semiconductor device is a lithography process, which generally comprises coating a photoresist film on a layer to be patterned, exposing and developing the photoresist film through a patterned mask to form a photoresist film pattern, and etching the layer with the photoresist film pattern serving as a mask.
To produce 4 G or 16 G DRAM semiconductor devices, a fine pattern as small as 0.18 .mu.m or less in size is required. However, the wet development process in current use has a difficultly in solving the problems of the breaking down of patterns and the allowance for the depth of focus on a practical topology. For this reason, a dry development process in combination with a silylation technique was developed and used in a masking process for 4 G and 16 G DRAM semiconductor devices.
In order to better understand the background of the invention, a description will be given of a conventional method for forming a photoresist film pattern through a silylation technique, in connection with some drawings.
First, a light beam is irradiated through a mask to a photoresist film 2 underlain by a layer 2 upon a semiconductor substrate 100, as shown in FIG. 1. The layer 2 is to be etched and consists of a photoresist material and a novolak resin.
FIG. 2 is a schematic view after the photoresist film 2 is subjected to a baking process, to make the irradiated regions 5 of the photoresist film 2 thermally cured whereas the regions which are not exposed to the light beam remain as they are.
FIG. 3 shows a silylation process in which a silylation agent, containing N--Si bond, such as hexamethyl disilazane or tetramethyldisilazane, is injected into the regions 5 exposed to the light beam. N--Si bond is so weak that it may be readily broken and react with R--O--H of the resin to form R--O--Si--(CH.sub.3).sub.3 (trimethylsilicon).
FIG. 4 shows a dry developing process utilizing O.sub.2 plasma, by which a silicon oxide 6 is formed in the trimethyl silicon bonded with the resin of the exposed regions whereas the unexposed regions of the photoresist film 2 are etched to form a photoresist film pattern 2'.
FIG. 5 is a schematic view after the layer 1 is etched to create a pattern 1', with the photoresist film pattern 2' serving as a mask.
As illustrated, this conventional method by which the exposed regions remain as a photoresist film pattern whereas the unexposed regions are removed, forms a pattern such as that of the general negative photoresist film.
The photoresist film pattern formed via the conventional silylation process is poor in profile because the boundaries between the exposed regions and the unexposed regions are not clear. In addition, light diffracts into the regions which are determined not to be exposed, generating a swelling phenomenon.
Typically, KrF (248 nm) or ArF (193 nm) is used as a light source. The photo acid compound (PAC) contained in the convention photoresist film depends on and varies with the light source.